The present invention generally relates to optical transceivers, and more particularly to an optical transceiver which uses a U-shaped optical bus as its transmission line and carries out a control dependent on packet collision in a local area network (LAN) which employs carrier sense multiple access with collision detection CSMA/CD).
The LAN which uses the U-shaped optical bus as its transmission line is virtually unaffected by external noise when compared to the conventional system which uses a metal cable as its transmission line. In addition, the restrictions on the design of the LAN system is relatively small.
FIG. 1 shows an example of a conventional LAN which uses the U-shaped optical bus as its transmission line and employs the CSMA/CD. In FIG. 1, a plurality of optical transceivers 80.sub.1 through 80.sub.N are arranged on a U-shaped optical bus 81. Each optical transceiver 80.sub.i is made up of a photocoupler 82.sub.i for making a photo-electric conversion in correspondence with the going and returning paths of the optical bus 81, and a coder/decoder part 83.sub.i which codes and decodes the electric signal, where i=1 to N.
In each coder/decoder part 83.sub.i, an output of the photocoupler 82.sub.i which is provided in the going path of the optical bus 81 is coupled to a receiving internal bus 87.sub.1 via a Manchester decoder 84, a clock adjusting circuit 85 and one channel of a serial interface adapter 86. The Manchester decoder 84 decodes a Manchester code included in an output signal of the photocoupler 82.sub.i which is provided in the going path of the optical bus 81 into an non-return-to-zero (NRZ) signal. The clock adjusting circuit 85 absorbs the difference in the accuracies of a clock in the optical bus 81 and a clock within the coder/decoder part 83.sub.i.
In addition, in each coder/decoder part 83.sub.i, a transmitting internal bus 87.sub.2 which forms a pair with the receiving internal bus 87.sub.1 is coupled to a clock adjusting circuit 88 via another channel of the serial interface adapter 86. An output of the clock adjusting circuit 88 is coupled to an input of the photocoupler 82.sub.i which is provided in the returning path of the optical bus 81, via a Manchester encoder 89 which codes the NRZ signal into the Manchester code. On the other hand, the receiving internal bus 87.sub.1 is coupled to a level shift circuit 90 which notifies a packet collision generated on the optical bus 81 to each data terminal by shifting the level at the receiving internal bus 87.sub.1. The internal buses 87.sub.1 and 87.sub.2 are coupled to each data terminal 92 via an AUI cable prescribed under ISO8802-3 and a transceiver module 94.
In the conventional LAN described above, packets which are simultaneously transmitted from a plurality of optical transceivers may collide on the optical bus 81. However, in each of the coder/decoder parts 83.sub.1 through 83.sub.N, two internal buses 87.sub.1 and 87.sub.2 are provided in order to avoid data collision on the internal bus, and the level shift circuit 90 is provided to notify the packet collision generated on the optical bus 81 to each data terminal. For this reason, there is a problem in that the hardware scale is large in the conventional LAN.